The research and development leading to the subject matter disclosed herein was not federally sponsored.
This invention relates to reinforcing materials for concrete and concrete structures so reinforced.
Concrete is one of the most common building materials. It is used in a wide variety of structures such as bridges, walls, floors, building supports, roadways, and runways among many others.
Concrete has excellent compressive strength, but is very poor in tensile strength. As a result, it is almost always necessary to reinforce a concrete structure if the structure will be exposed to tensile stresses such as those generated by a bending load. A very common way of providing this reinforcement is to incorporate metal (usually steel) reinforcing bars into the concrete. Steel reinforcing bars can provide a great improvement in tensile strength to the concrete structure.
Unfortunately, steel reinforcing bars corrode over time when exposed to water. This corrosion is accelerated if the steel is exposed to salts, as are often used in colder climates to melt snow and ice from the road surface. The concrete tends to provide some protection from water and salts, but over time cracks develop in the concrete and these materials are able to seep through the cracks to the embedded steel. As the steel begins to corrode, it expands due to the formation of rust layers. This expansion causes further cracking in the concrete, thereby accelerating the decay of the concrete structure.
To avoid this corrosion problem, certain pultruded composites have been tried. These composites include a thermoset resin that serves as a matrix into which longitudinal fibers, usually glass but sometimes of other materials, are embedded.
These thermoset composites solve the corrosion problem, but have other significant drawbacks. The most significant of these is that there is no practical way that these thermoset composites can be formed into a variety of shapes. Steel reinforcing bars are commonly bent, twisted or formed into rings in order to accommodate them to the needs of a particular construction project. This is often done on-site, but can also be done as part of the rebar manufacturing process. Pultruded thermoset composites are not formable once the thermosetting resin matrix is cured. Thus, on-site forming is not an option with the thermoset composites. Even in-factory forming is difficult. The pultrusion process is mainly adapted for making straight composites of constant cross-section. Any forming that is done must take place during a brief time window between the time the resin is applied to the reinforcing fibers and cured to a viscosity that it will not run off and the time the resin is fully cured. This short time window makes forming very difficult and expensive to accomplish for a thermoset composite.
A second major shortcoming of thermoset composites is that they are difficult to key into the concrete. Steel rebars often have raised or indented sections that are molded or stamped onto the surface of the bar. These sections permit the bar to be mechanically interlocked into the concrete. Thermoset composites, on the other hand, usually have a constant cross-section due to the nature of the pultrusion process. Post-forming methods for providing surface features such as stamping are not suitable because the thermoset composites tend to be brittle and have poor impact resistance. The stamping process tends to break the embedded fibers and weaken the composite. Sometimes overmoldings are used to provide a raised surface for keying into the concrete. However, the bond between the overmolding and the composite is often weaker than the concrete matrix and thus provides little benefit.
In addition, thermoset composites suffer from poor elongation (on the order of 1% at break), poor impact resistance and brittleness. They also are quite expensive, mainly due to slow production rates.
It would therefore be desirable to provide an alternative to steel and thermoset composite reinforcing bars for concrete structures.
In one aspect, this invention is a reinforcing bar (rebar) comprising a composite of a plurality of longitudinally oriented reinforcing fibers embedded in a matrix of a thermoplastic resin.
The reinforcing bar of this invention solves many of the problems associated with steel and thermoset composite rebars. The rebar of this invention does not corrode due to exposure to water and/or common salts. The rebar of the invention is readily formed into a great many shapes and configurations. As a result, it is easily formed into shapes that enable it to key into concrete, forming a mechanical interlock with the concrete that improves the reinforcing effect. This forming can be done easily on-site if desired. The reinforcing bar of the invention is often capable of being manufactured at higher rates than pultruded thermoset composites. As a result, the rebar of the invention can be less expensive and perform better than thermoset composite rebars.
In a second aspect, this invention is a concrete structure comprising a reinforcing bar embedded in a concrete matrix, said reinforcing bar comprising a composite of a plurality of longitudinally oriented reinforcing fibers embedded in a matrix of a thermoplastic resin.